Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed upon reflection.
Fourier domain OCT (FD-OCT) currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diversity detection. It has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum-encoded FD-OCT, are not available.
In swept source OCT, the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its frequency tuning speed and accuracy.
High speed frequency tuning for OCT swept sources is especially relevant to in vivo imaging where fast imaging reduces motion-induced artifacts and reduces the length of the patient procedure. It can also be used to improve resolution.
The swept sources for OCT systems have typically been tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. A tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA) that is located within a resonant cavity, and a tunable element such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest tuning speed lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. The use of micro-electromechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass, high mechanical resonant frequency deflectable MEMS membranes that have the capacity for high speed tuning.
Certain tradeoffs in laser design, however, can be problematic for OCT systems. Generally, shorter laser cavities translate to higher potential tuning speeds, since laser oscillation must build up anew from spontaneous emission when the laser is tuned. Thus, round-trip travel time for the light in the laser cavities should be kept low so that this build up occurs quickly. Short laser cavities, however, create problems in terms of the spectral spacing of the longitudinal cavity modes of the laser. That is, lasers can only produce light at frequencies which are integer multiples of the cavity mode spacing since the light must oscillate within the cavities. Shorter cavities result in fewer and more widely spaced modes. This results in greater mode hopping noise as the laser is tuned over these discrete cavity modes. So, when designing an OCT laser, there is typically a need to choose between low noise and high speed.
Research with swept tunable lasers has shown that when they are operated at high sweep rates they tend to operate in a mode locked regime. In a mode locked regime optical power of the laser varies on a time scale of the cavity roundtrip time as one or more optical pulses travel in the laser cavity, as is found in a traditional mode locked laser. The pulse repetition rate is close to the laser cavity roundtrip time or to a typically small, say a factor of 2 to 10, multiple. Since this mode locking arises from frequency tuning of the laser, it is termed swept mode locking.
This swept mode locked regime can have the effect of actually facilitating the high-speed tuning of the laser. A four-wave mixing effect red shifts the wave in the laser cavity. This facilitates the tuning to lower optical frequencies. See A. Bilenca, S. H. Yun, G. J. Tearney, and B. E. Bouma, “Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications”, OPTICS LETTERS/Vol. 31, No. 6, Mar. 15, 2006.
Problems, however, often arise when tuning to higher optical frequencies and also during very high speed tuning. Generally, this tuning tends to be more unstable. Some of these instabilities probably result from the fact that the laser cavity is changing through the process of tuning, and thus the characteristics that instigate the swept mode locking also change. As a result, the lasers can flip between different swept mode locked regimes during a single frequency scan of the tunable laser. For example, during the sweep, the number of pulses circulating in the cavity can change, causing the lasers to behave chaotically and unpredictably as they move between the different regimes. The different regimes can further result in different performance characteristics as the tunable lasers relate to the OCT systems in which they operate.